Silicon carbide (SiC) has excellent heat resistance and mechanical strength and is physically and chemically stable, and therefore is attracting attention as an environment-resistant semiconductor material. In recent years, demands for an SiC single crystal substrate as a substrate for a high-frequency high-voltage resistant electronic device or the like are increasing.
In the case of manufacturing, for example, an electric power device or a high-frequency device by using an SiC single crystal substrate, it is common to epitaxially grow an SiC thin film on the substrate by using a method referred to as a thermal CVD process (thermal chemical vapor deposition process) or directly implant a dopant by an ion implantation process. The latter case requires annealing at a high temperature after the implantation and for this reason, formation of a thin film through epitaxial growth is employed in many cases.
At present, the predominant diameters of an SiC substrate are 3 inches and 4 inches (76 mm and 100 mm), and therefore epitaxial growth is performed on such a substrate, but from the standpoint of, for example, reducing the density of defects such as basal plane dislocation or increasing the yield of a substrate from an SiC ingot, a substrate not having a conventional off-angle of 8° but having an off-angle of about 4° or less is used. In the case of epitaxial growth on a substrate having such a small off-angle, the ratio (C/Si ratio) of the number of carbon atoms to the number of silicon atoms in the raw material gas flowed during growth is generally set to be lower than the conventional ratio. The ratio above is set so as to avoid the fact that as the off-angle becomes smaller, the number of steps on the surface is decreased and this leads to less occurrence of step-flow growth and susceptibility to an increase in step bunching or epitaxial defect. Furthermore, studies are being made to carry out the pre-growth treatment not in conventional hydrogen gas but in hydrogen gas containing silicon or carbon and Thereby more improve the surface state of the epitaxial film on a low off-angle substrate (Non-Document 1). In Non-Patent Document 1, a 4°-off SiC substrate is subjected to pretreatment etching in an atmosphere of hydrogen, hydrogen+propane or hydrogen+silane and then to epitaxial growth, and by performing the pretreatment etching in hydrogen+silane, the surface state of the epitaxial film is improved as compared with the conventional epitaxial film. This is due to the fact that etching in the Si-rich state enables not only removal of the surface modification layer but also reduction and stabilization of the surface energy and therefore, when growth is started, generation of step bunching or defect involved in surface energy relaxation is suppressed. However, decomposition of silane causes production of many kinds of compounds taking the form of SixHy and since the ratio thereof is considered to determine the etching state, the surface state after etching cannot be easily controlled with good reproducibility. Furthermore, when the silane concentration is locally increased, silanes combine with each other to form an Si droplet and remain on the substrate, giving rise to a surface defect and in turn, worsen the surface state.
Accordingly, despite an SiC epitaxial growth substrate expected to find application in a device in the future, growth with a low C/Si ratio and pretreatment etching in a hydrogen+silane atmosphere are necessary for obtaining a more excellent surface state than in the conventional film by using a substrate having not a conventional off-angle of 8° but having an off-angle of about 4° or less. However, it is not easy to ensure a stable etching state by controlling the decomposition of silane in this pretreatment etching, and moreover, because of a problem of Si droplet that is generated when the silane concentration is locally increased, stable formation of an epitaxial film with good surface state is difficult. Incidentally, Patent Document 1 describes a technique of incorporating hydrogen chloride (HCl) into a carrier gas of the reaction gas or incorporating chlorine (Cl) into a silicon (Si) raw material gas, during epitaxial growth. This is aiming at enhancing the uniformity of the thickness or doping density of the epitaxial film by etching the epitaxial film with chlorine incorporated or by accelerating the decomposition of the Si raw material gas by chlorine. That is, Patent Document 1 is not related to the pretreatment of epitaxial growth.
Patent Document 2 relates to a production process of an SiC single crystal wafer and describes a technique of applying a surface treatment (for example, chemical-mechanical polishing or hydrogen etching) to the substrate surface before CVD growth and etching the substrate surface in the course of raising or lowering the temperature. However, specific conditions for etching are not disclosed.
Patent Document 3 relates to etching of a substrate surface by an in-situ deposition process. The problem in Patent Document 3 is that a chemical etching substance (such as HCl, Cl2, HF and HBr) and a deposition process gas containing silicon and hydrogen cannot be used together except for under limited conditions because of a safety problem or are incompatible. In order to solve this problem, a technique of reacting a hydrogen-containing gas and a chemical etching substance to form a quasi-stable chemical species adjacently to the wafer surface is described. As for the quasi-stable chemical species, it is indicated that various species vigorously changing in the reaction path of growth and etching are produced and each species has a short life. However, specific conditions enabling realization of high-quality epitaxial growth are not disclosed. In addition, since various quasi-stable chemical species can be produced, an adverse effect may occur on the substrate surface.